The objective of this multicenter observational study is to ascertain whether in utero magnesium sulfate (MgSO4) exposure is associated with a reduction in the incidence of intraventricular hemorrhage and /or cerebral palsy in extremely low birth weight infants. Data concerning whether MgSO4 was given to the mother and, if so, the timing and dosage, will be collected and linked to data from two ongoing studies: Survey of Morbidity and Mortality Among Very Low Birth Weight Infants (401 - 1500 grams) and Follow-Up of Extremely Low Birth Weight Infants (401 - 1000 grams). This link will allow assessment of the impact, if any, of antenatal MgSO4 on the neurologic outcome of infants with birth weights from 401 through 1000 grams.